1. Field of the Invention
The present invention relates to an image forming apparatus, an image forming method and a process cartridge.
2. Discussion of the Background
In recent years, with the advance of the size reduction of an image forming apparatus, image bearing members have been decreased in size. Also, high speed operation and maintenance-free operation have been demanded and thus an image bearing member having a high durability has been desired. From this point of view, organic photoconductors (image bearing members) are generally soft because the surface layer thereof is mainly made of a low molecular weight charge transport material and an inert polymer. When such an organic photoconductor is repetitively used in the electrophotography process, the organic photoconductor tends to be abraded under mechanical stress by a developing system or a cleaning system.
In addition, in accordance with size reduction of toner particles for improving image quality, the rubber of a cleaning blade is hardened and the contact pressure between an image bearing member and a cleaning blade is increased to improve the cleaning property. This accelerates the abrasion of the image bearing member. Such abrasion of an image bearing member causes deterioration of electric characteristics, such as sensitivity and chargeability, which leads to production of abnormal images, such as a decrease in image density and background fouling. When an image bearing member is locally damaged by abrasion, the damaged portion causes streaks on an image resulting from bad cleaning performance on the image bearing member. Currently, this abrasion or damage is a controlling factor of the lifetime of an image bearing member and once an image bearing member has such abrasion or damage, the image bearing member must be replaced immediately to sustain image quality and performance.
With regard to charging of an image bearing member, a contact type charging system is adopted in which a charging unit (e.g., a charging roller) formed of a metal core covered with an elastic electroconductive member is brought into contact with an image bearing member and a driving voltage is applied to the charging unit (Refer to unexamined published Japanese Patent Applications No. (hereinafter referred to as JOP) S63-149668 and H01-267667).
However, in this contact type charging system, an image bearing member is charged while in contact with a charging unit. Therefore, toners, external additives, dust, etc. that have slipped through a cleaning blade are attached to the charging unit (charging roller). This contamination on a charging unit causes production of uneven images. Also the photosensitive layer is unevenly abraded by the contamination attached to the charging unit. Thus, the lifetime of such an image bearing member tends to be shortened.
With regard to contamination on a charging unit, an AC charging system has a relatively large margin in comparison with a DC charging system. It is thus preferred to select an AC charging system in consideration of the stability of images over time. The contamination on a charging unit may be decreased by the selection of an AC charging system, however, since the charging unit is in direct contact with the image bearing member, the margin has not reached the level to which an image bearing member having a desired lifetime is provided.
To avoid the problems mentioned above, JOPs 2001-194868 and 2002-55508 describe an image forming apparatus having a non-contact type charging device which can be disposed in the vicinity of an image bearing member by a spacer provided at both ends of the charging device. By this structure, an image bearing member can be uniformly charged and ozone is less produced and also the charging device is less contaminated by residual toner remaining on the image bearing member.
The disposition in the vicinity of an image bearing member represents a state in which there is a minute gap between a non-contact type charging device and an image bearing member. The gap is preferably from 10 to 100 μm and more preferably from 20 to 50 μm. Thereby, charging unit fouling can be reduced but it is not sufficient to produce a quality image without image deficiency.
For example, JOP 2006-154387 describes an image forming apparatus capable of producing images without image deficiency caused by contamination on a charging device (roller) by specifying an external additive added to a toner in the case of a non-contact type charging device (roller) disposed in the vicinity of an image bearing member.
In each teaching described above, an image bearing member having a high durability is obtained. The surface of such an image bearing member is hard and tough for abrasion, however, has an opposite action, namely, this type of image bearing member is somewhat inferior to a typical image bearing member with regard to the margin for abrasion by a cleaning blade. Thus, the cleaning performance deteriorates according to this phenomenon especially when quality images are pursued with a toner having a small particle diameter.